1. Field of the Invention
The present invention generally relates to a filter using a piezoelectric material, and more particularly, to a filter that includes interdigital transducers (IDTs) formed on a piezoelectric substrate.
2. Description of the Related Art
In recent years, filters that are formed with surface acoustic wave devices each having interdigital transducers on a piezoelectric substrate are used as bandpass filters for television systems with frequency ranges of 30 MHz to 400 MHz, and as RF filters for portable telephone devices with 800 MHz or 1–9 GHz frequency ranges. An IDT has a pair of comb-like electrodes. Each of the comb-like electrodes includes a bus bar and electrode fingers. One end of each of the electrode fingers is connected to the bus bar, and the other end of each of the electrode fingers is open. The pair of comb-like electrodes is arranged so that the electrode fingers of one of the comb-like electrodes become adjacent to, or overlap with, the electrode fingers of the other one of the comb-like electrodes at predetermined intervals. When an AC voltage is applied to the pair of comb-like electrodes, a surface acoustic wave is generated. The surface acoustic wave has frequency characteristics. Utilizing the frequency characteristics, a filter with desired frequency characteristics can be realized.
FIG. 1 illustrates a conventional filter utilizing surface acoustic waves. The same type of filter as the filter shown in FIG. 1 is disclosed in Japanese Patent Application Publication No. 10-41778. The filter shown in FIG. 1 includes a first IDT 10, a ground electrode 20, and a second IDT 30. The first IDT 10, the ground electrode 20, and the second IDT 30 are formed on a piezoelectric substrate, and are adjacent to one another in the surface acoustic wave propagation direction. The ground electrode 20 is interposed between the first IDT 10 and the second IDT 30, and functions as a shield electrode. The first IDT 10 serves as an input electrode (or an output electrode), while the second IDT 30 serves as an output electrode (or an input electrode). The ground electrode 20 prevents electromagnetic coupling between the IDT 10 and the IDT 30. Also, the ground electrode 20 is obliquely arranged so that surface acoustic waves generated from the IDT 10 (or the IDT 30) can be prevented from being reflected and returning to the IDT 10 (or the IDT 30).
The IDT 10 has a pair of comb-like electrodes 10a and 10b. The comb-like electrode 10a has a bus bar 12a and electrode fingers 14a. Likewise, the comb-like electrode 10b has a bus bar 12b and electrode fingers 14b. The open end of each of the electrode fingers 14a faces the open end of each corresponding one of the electrode fingers 14b. The overlapping parts of the neighboring electrode fingers 14a and 14b (the electrode finger overlapping parts), or the electrode finger parts facing each other, contribute to excitation of surface acoustic waves. In the structure shown in FIG. 1, the electrode finger pattern formed by the electrode fingers 14a and 14b is weighted. More specifically, the electrode finger pattern shown in FIG. 1 is weighted by apodization. In the apodized electrode finger pattern, the widths of the electrode finger overlapping parts (the widths will be hereinafter referred to as the aperture lengths) vary in the surface acoustic wave propagation direction. At either end part of the IDT 10, the aperture lengths are comparatively small. These parts are generally referred to as minute overlapping parts. The IDT 10 has greater aperture lengths in the center region. The aperture lengths are proportional to excitation intensity. Accordingly, surface acoustic wave has higher intensity in the center of the IDT 10 than the intensity at either end region of the IDT 10. The frequency characteristics can be varied by adjusting the apodization weighting.
The IDT 30 also includes a pair of comb-like electrodes, but these comb-like electrodes are not weighted. Accordingly, the aperture lengths in the IDT 30 are uniform. This type of IDT is called a normal IDT.
The filter with the above structure functions as a bandpass filter.
The filter shown in FIG. 1 and disclosed in Japanese Patent Application Publication No. 10-41778, however, cannot adequately attenuate the frequency components in the stop bands.